An international group of more than 260 scientists has produced one of the most stringent tests to date for the existence of sterile neutrinos, which are theorized particles related to the three known types, or “flavors,” of neutrinos but that are not directly detectable. Sterile neutrinos are a candidate for mysterious dark matter, which so far has only been observed via its gravitational effects and makes up a whopping 85 percent of the total mass of the universe.
The scientists – from the Daya Bay collaboration in China and the MINOS+ team at the Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) – are reporting results in Physical Review Letters ruling out changes, or “oscillations,” from a known flavor of neutrino into a sterile neutrino as the primary explanation for unexpected observations from recent experiments. DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) leads U.S. involvement in the Daya Bay experiment.
The combined analysis reported by Daya Bay and MINOS+ ruled out the specific kind of sterile neutrino behavior that would explain anomalous results reported by some previous experiments – those results had held open the possibility of sterile neutrinos’ existence. And the analysis also looked for other sterile neutrino signatures with never-before-achieved sensitivity, yielding some of the most stringent limits on the existence of these particles to date.
“This joint effort very effectively tackles a fundamental problem in physics,” said Daya Bay co-spokespersons Kam-Biu Luk, a faculty senior scientist at Berkeley Lab and physics professor at UC Berkeley, and Jun Cao of the Institute of High Energy Physics in Beijing, in a joint statement. “While there is still room for a sterile neutrino to be lurking in the shadows, we have significantly shrunk the available hiding space.”
There are three known types of neutrinos: electron, muon and tau. Daya Bay uses eight identically designed detectors to precisely measure how electron neutrinos emitted by six nuclear reactors in China “disappear” as they morph into other types. Similarly, MINOS+ studies the disappearance of muon neutrinos produced by a Fermilab accelerator and propagating to an underground detector in northern Minnesota 735 kilometers away.
Neutrinos are elementary particles that, like electrons, cannot be broken down into smaller components. They are unlike any other particle known to exist in that they are able to penetrate extremely large amounts of matter without stopping. If a neutrino is shot from the surface of Earth toward its center, there is a very large probability that it will emerge intact on the other side.
About two decades ago, scientists found that they can morph from one type into another through a phenomenon called “neutrino oscillation,” a discovery that was awarded the 2015 Nobel Prize in physics. For instance, a neutrino created as an electron type traveling through space can later be identified as a muon type or tau type.
Even though the vast majority of accumulated data to date can be explained by three known neutrinos, a few experiments have reported anomalous observations suggesting the existence of additional types. Among these are the LSND experiment at the Los Alamos National Laboratory and the MiniBooNE experiment at Fermilab.
In both of those experiments, detectors were exposed to a beam of muon neutrinos, and there were reported excesses of electron neutrino candidate events beyond what would be expected from oscillations involving only the three known neutrinos. One explanation cited for these excesses was the presence of sterile neutrinos – their oscillations with the three known neutrino flavors would provide a unique pathway to establish their existence.
However, the new results from Daya Bay and MINOS+ now question this possibility as an explanation of the LSND and MiniBooNE results.
“The stakes are high; if this tantalizing interpretation of the anomalous results was confirmed, a revolution in physics would ensue,” said Daya Bay scientist Pedro Ochoa-Ricoux, associate professor of physics and astronomy at UC Irvine and a former Chamberlain fellow of the Physics Division at Berkeley Lab.
“Sterile neutrinos would become the first particles to be found outside the Standard Model, our current best theory of elementary particles and their interactions. They could also be a candidate for dark matter and might have important consequences in cosmology,” Ochoa added.
“This close collaboration of MINOS+ and Daya Bay scientists enabled the combination of two complementary world-leading constraints on muon neutrinos and electron antineutrinos disappearing into sterile neutrinos,” said Alexandre Sousa, associate professor of physics at the University of Cincinnati and one of the MINOS+ scientists who worked on the analysis. The disappearance of both particles needs to occur if electron (anti)neutrinos are to appear in a muon (anti)neutrino source via sterile oscillations. “So the combined result is a very powerful probe of the sterile neutrino hints we have to date.”
The neutrino disappearance measurements by Daya Bay and MINOS+ are now so precise that they essentially rule out explaining the combined anomalous observations from LSND, MiniBooNE and other experiments with oscillations involving one or more sterile neutrinos, according to Ochoa-Ricoux.
“We would all have been absolutely thrilled to find evidence for sterile neutrinos, but the data we have collected so far do not support any kind of oscillation with these exotic particles,” he said.
“The two experiments use multiple detectors with well-understood uncertainties and have collected an unprecedentedly large number of events. Requiring consistency between the data sets of the two experiments provides a very rigorous test of sterile neutrino existence,” said MINOS+ spokespersons Jenny Thomas and Karol Lang jointly in a statement. Thomas is a professor at University College London, and Karol Lang is a professor at the University of Texas at Austin.